1. Field of the Invention
This invention relates, in general, to a current source and more particularly to a current source having a relatively small capacitor and resistor connected so as to compensate for oscillations while maintaining a high impedance.
2. Background Art
Some previously known current source circuits comprise a first NPN transistor having a base connected to a bias voltage and its collector and emitter coupled between supply voltages. A second NPN transistor has its base connected to the collector of the first transistor and its collector and emitter coupled between the supply voltages. PNP transistors are coupled between one of the supply voltages and the collectors of the first and second transistors to supply current thereto.
A feedback loop including the first and second NPN transistors and the PNP transistors possibly would cause oscillations that would appear in the current output. One common technique for stabilizing the circuit was to couple a relatively large capacitor, i.e. 100 picofarad, between the base of the first transistor and ground. Another compensation technique was to couple a capacitor between the base and collector of the first transistor, which gives the Miller multiplied capacitance at the base of the transistor.
However, since the emitter of the second transistor was grounded, the impedance at the collector of the first transistor was not very high, and the pole created was not sufficiently low in frequency because the gain was too low.
Therefore, what is needed is a current source circuit wherein the area consumption of an on-chip capacitor is reduced while giving the Miller multiplication effect by way of a higher impedance at the collector of the first transistor.